i'm having same issue with the E6420 chip.
3.2ghz seems my limit. L702C560 stepping, on FuZion and g/byte's N680SLI-DQ6 board, F5b bios.

can boot and superpi at 500x6 (3ghz), but get it over 3.2ghz (500x7 or 415x8) and it dies. i guess 3.2 aint toooo bad tho.